1. Field of the Invention
The present invention relates to technology which is effective when applied to an optical waveguide board that includes an optical waveguide for transmitting an optical signal between light receiving and emitting elements (signifying a light emitting element and/or a light receiving element).
2. Description of the Related Art
JP-A-2002-174744 discloses a technique wherein mirrors and via holes are formed in an optical wiring layer (optical waveguide) which is formed on a glass substrate by the use of an optical wiring layer material (for example, polyimide). Concretely, the mirrors which reflect the optical signal of optical elements (light receiving and emitting elements) into the optical wiring layer, and the via holes which are connected to the optical elements are formed in the optical wiring layer.
For example, an electronic circuit which is so configured that a semiconductor device is mounted on a wiring board (electric board), the rate of signal transmission becomes high on account of rise in the operating speed (operating frequency) of the semiconductor device such as a CPU. In consequence, noise which causes malfunctions is liable to occur from the electric wiring of the wiring board.
In this regard, there has been studied an optical/electrical hybrid device employing a technique wherein part of the electric wiring which constitutes the electronic circuit is substituted from the electric wiring into optical wiring based on an optical waveguide, so as to transmit a signal by light receiving and emitting elements. This technique uses an optical waveguide board including a first clad layer, a core layer which is formed thereon and which transmits the optical signal, a second clad layer which is formed on the first clad layer so as to cover the core layer, and mirrors which change the propagation directions of the optical signal by reflections.
Now, an example of the optical waveguide board studied by the inventor will be explained with reference to FIG. 1A through FIG. 3B. FIGS. 1A through 3B are explanatory views schematically showing the essential portions of the optical waveguide board (one end side of the optical waveguide) under a manufacturing process studied by the inventor, and FIGS. 1A, 2A and 3A are plan views, while FIGS. 1B, 2B and 3B are sectional views taken along lines X-X in FIGS. 1A, 2A and 3A, respectively. Incidentally, one end side of the optical waveguide is illustrated in the drawings and will be explained, and the other end side is similar and shall therefore be omitted from the explanation.
First, as shown in FIGS. 1A and 1B, wiring patterns 103 having connection pads 102 are formed on a wiring board which is formed with electric wiring (and which serves as the substrate 101 of an optical waveguide board). Incidentally, the diameter of each connection pad 102 within a plane is, for example, 90 μm, and the pitch between the adjacent wiring patterns 103 is, for example, 125 μm.
Subsequently, a resin to become a clad layer 104 is formed and hardened on the substrate 101 so as to cover the wiring patterns 103. Subsequently, a resin which has a refractive index larger than that of the clad layer 104 and which becomes a core layer 105 is formed and hardened on the clad layer 104, and it is thereafter patterned so as to extend as the core layer 105. Subsequently, the resin which is identical to that of the clad layer 104 and which becomes a clad layer 106 is formed and hardened on the clad layer 104 so as to cover the core layer 105. Incidentally, the width of each pattern of the clad layer 105 within the plane is, for example, 35 μm, and the pitch between the adjacent patterns of the clad layer 105 is, for example, 250 μm.
In this way, the optical waveguide which is configured of the core layer 105, and the clad layers 104 and 106 holding the core layer 105 therebetween, is formed in the surface layer of the substrate 101 (wiring board). Here, the core layer 105 and the clad layers 104 and 106, namely, the optical waveguide are/is made from, for example, an epoxy type resin or a polyimide type resin, which is a resin excellent for optical characteristics as the optical waveguide. Besides, the resins which are used for the core layer 105 and the clad layers 104 and 106 shrink due to the hardening. Therefore, especially the core layer 105 is formed having a certain degree of design margin.
Subsequently, as shown in FIGS. 2A and 2B, at the end part of the optical waveguide, a smooth surface 107 which inclines to the extending direction of the core layer 105 (a direction in which an optical signal is transmitted) is formed by, for example, a dicer, and a metal film 108 is thereafter formed on the smooth surface 107.
The metal film 108 serves as a mirror which changes the propagating direction of light between a light receiving or emitting element and the optical waveguide, by reflection. Therefore, the formation of the smooth surface 107 on which the metal film 108 is to be formed becomes important for the transmission efficiency of the optical signal. That is, in forming the smooth surface 107 by the dicer, a high precision is required.
Subsequently, through-holes 109 are formed in the clad layers 104 and 106 overlying the wiring patterns 103, so as to expose the connection pads 102 of the wiring patterns 103, by employing a photolithographic technique and an etching technique by way of example. Incidentally, each through-hole 109 has a diameter of, for example, 80 μm, and it is formed at a precision of, for example, ±2 μm.
Subsequently, as shown in FIGS. 3A and 3B, the through-holes 109 are filled up with a conductive material (for example, copper) by employing, for example, an electroless plating technique, thereby to form vias 110. Thus, the optical waveguide board 111 is substantially completed.
Incidentally, it has been revealed as the result of study that, in the case where the through-holes 109 are filled up with the conductive material by the electroless plating technique, they are insufficiently filled up when the aspect ratio (diameter/height) of each of them is one or less. Besides, it is considered that, in a case where the through-holes 109 are configured by forming through-holes in each of the clad layers 104 and 106 by patterning, and then joining the corresponding through-holes of the respective clad layers 104 and 106, a higher positional precision (patterning precision) will be required.
Thereafter, a light receiving element and a light emitting element are respectively mounted on both the end sides of the optical waveguide on the optical waveguide board 111. In FIGS. 3A and 3B, the connection terminal 113 of a light receiving/emitting element 112, which is the light receiving element or the light emitting element, is electrically connected with the via 110 on the side of the end part of the optical waveguide. Besides, the light receiving or emitting element 112 mounted on the optical waveguide board 111 is electrically connected with the wiring pattern 103 through the via 110, and it receives a control signal from this wiring pattern 103.
Thus, an optical signal 114 for the light receiving or emitting element 112 can be transmitted through the smooth surface 107 (metal film 108), between the light receiving or emitting element 112 and the core layer 105 which constitutes the optical waveguide. More specifically, an optical signal emitted from the light emitting portion of the light emitting element is reflected by a mirror on one end side of the optical waveguide and then transmitted through the core layer 105 of the optical waveguide, and it is further reflected by a mirror on the other end side of the optical waveguide and then received by the light receiving portion of the light receiving element.
Here, in case of considering the transmission efficiency of the optical signal, the positional precisions of the optical waveguide (core layer 105), the smooth surface 107 on which the metal film 108 to serve as the mirror is formed, and the via 110 become important in the optical waveguide board 111.
By way of example, let's consider a case where positional deviations have appeared in the smooth surface 107 and the via 110. The light receiving or emitting element 112 which is mounted on the optical waveguide board 111 has its connection terminal 113 connected with the via 110, so that the position of the light receiving or emitting portion of the light receiving or emitting element 112 is determined with reference to the via 110. When the positional deviations have appeared in the smooth surface 107 and the via 110, also the position of the light receiving or emitting portion of the light receiving or emitting element 112 relative to the smooth surface 107 deviates.
Thus, part of the optical signal emitted from the light emitting portion of the light emitting element and then reflected by the mirror, fails to enter the core layer 105, and the transmission efficiency of the optical signal in the optical waveguide board 111 lowers. Besides, part of the optical signal exiting from the core layer 105 is reflected by the mirror without being received by the light receiving portion of the light receiving element, and the transmission efficiency of the optical signal in the optical waveguide board 111 lowers.